Isaac made a covenant with the people of the land, when he sojourned in the land of the Philistines. He noticed that they turned their faces away from him. He went away from them in peace, and Abimelech and all his magnates came after him. He said to them: Ye turned aside your faces from me, and now ye come unto me, as it is said, "And Isaac said unto them, Wherefore are ye come unto me, seeing ye hate me?" (Gen. 26:27).
"And they said, We saw plainly that the Lord was with thee" (Gen. 26:28). They said: We know that the Holy One, blessed be He, will give to thy seed in the future all these lands; make a covenant of an oath with us, that thy seed will not take possession of the land of the Philistines. He made a covenant of an oath with them. What did Isaac do?
He cut off one cubit of the bridle of the ass upon which he was riding, and he gave it to them that it might be in their hands for a sign of the covenant of the oath.